1. Field of the Invention
The present invention relates to a contact electrode for a gallium nitride-based compound semiconductor device and a method for forming the same, and more specifically to an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor device and a method for forming the same.
2. Description of Related Art
An n-type contact electrode for a gallium nitride-based compound semiconductor device has realized a relatively low specific contact resistance by using a multi-layer or alloy electrode and an n-type GaN contact layer. For example, Japanese Patent Application Pre-examination Publication No. JP-A-07-045867, the content of which is incorporated by reference in its entirety into this application, and U.S. Pat. No. 5,563,422 claiming Convention Priorities based on eight Japanese patent applications including said Japanese patent application, the content of which is incorporated by reference in its entirety into this application, disclose that an alloy of Ti (titanium) and Al (aluminum) and a multi-layer film of Ti and Al are preferred as the n-type contact electrode. This will be called a xe2x80x9cfirst prior art examplexe2x80x9d hereinafter.
Referring to FIG. 1, there is shown a diagrammatic sectional view of the n-type contact electrode of the first prior art example. As shown in FIG. 1, the n-type contact electrode of the first prior art example. As shown in FIG. 1, the n-type contact electrode has a construction in which a Ti layer 102 and an Al layer 103 are deposited on an n-type GaN contact layer 101 in the named order. In this construction, an ohmic characteristic is obtained by annealing at a temperature of not less that 400xc2x0 C.
Referring to FIGS. 2A to 2D, there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the first prior art example. This method is disclosed by A. T. Ping et al, xe2x80x9cOhmic Contacts to n-type GaN Using Pd/Al Metallizationxe2x80x9d, Journal of Electronic Materials, Vol. 25, No. 5, 1996, pp.819-824, the content of which is incorporated by reference in its entirety into this application. This method will be called a xe2x80x9csecond prior art examplexe2x80x9d hereinafter.
In the method of the second prior art example, first, an n-type GaN contact layer 101 is etched by a dry-etching as shown in FIG. 2A, and an ashing processing is conducted by using an oxygen plasma as shown in FIG. 2B, and thereafter, as a pre-processing, an etching is conducted by using a hydrochloric acid aqueous solution as shown in FIG. 2C, and then, the Ti layer 102 and the Al layer 103 are deposited on the n-type GaN contact layer 101 in the named order as shown in FIG. 2D. Finally, a rapid thermal annealing (abbreviated to xe2x80x9cRTAxe2x80x9d) is conducted at a temperature of 650xc2x0 C. for 30 seconds.
In this case, a specific contact resistance of 6xc3x9710xe2x88x926 xcexa9cm2 is obtained. This specific contact resistance does not greatly change even if the RTA temperature changes in the range of 550xc2x0 C. to 750xc2x0 C. However, if the RTA temperature is less than 550xc2x0 C. or if no annealing is conducted, the ohmic characteristics cannot be obtained.
Furthermore, Japanese Patent Application Pre-examination Publication No. JP-A-07-221103, the content of which is incorporated by reference in its entirety into this application (an English abstract of JP-A-07-221103 is available from the Japanese Patent Office and the content of the English abstract of JP-A-07-221103 is also incorporated by reference in its entirety into this application), discloses an electrode structure which has improved the electrode structure of the first prior art example. This will be called a xe2x80x9cthird prior art examplexe2x80x9d hereinafter.
In the electrode structure of this third prior art example, after a double layer metal film of Ti and Al is formed on an n-type semiconductor layer with Ti being in contact with the n-type semiconductor layer, or after an alloy film of Ti and Al is formed on the n-type semiconductor layer, a metal having a melting point higher than that of Al is deposited. The third prior art exemplifies Au, Ti, Ni, Pt, W, Mo, Cr and Cu as metal having a melting point higher than that of Al, and mentions that Au, Ti and Ni are particularly preferable.
Referring to FIG. 3, there is shown a diagrammatic sectional view of the n-type contact electrode of the third prior art example. As shown in FIG. 3, the n-type contact electrode has a construction having a Ti layer 102, an Al layer 103, an Ni layer 104 and an Au layer 105, which are deposited on an n-type GaN contact layer 101 in the named order. In this example, an ohmic characteristics is obtained by annealing at a temperature of not less than 400xc2x0 C., similarly to the first prior art example.
In the contact electrode structure of the third prior art example, the Ni layer 104 prevents aluminum from separating out to a surface and also suppresses oxidation of aluminum. Therefore, it is advantageous in that a bonding wiring formed onto the Au layer 105 becomes difficult to be peel off.
Referring to FIGS. 4A and 4B, there are shown diagrammatic sectional views for illustrating one method for forming the electrode structure of the third prior art example. This method is disclosed by Z. Fan et al, xe2x80x9cVery low resistance multilayer ohmic contact to n-GaNxe2x80x9d, Applied Physics Letters, Vol. 68, No. 12, Mar. 18, 1996, pp.1672-1674, the content of which is incorporated by reference in its entirety into this application. This method will be called a xe2x80x9cfourth prior art examplexe2x80x9d hereinafter.
In the method of the fourth prior art example, first, an n-type GaN contact layer 101 is etched by a dry-etching as shown in FIG. 4A, and then, a Ti layer 102, an Al layer 103, an Ni layer 104 and an Au layer 105 are deposited on the n-type GaN contact layer 101 in the named order as shown in FIG. 4B. Finally, the RTA processing is conducted at a temperature of 900xc2x0 C. for 30 seconds.
In this electrode structure, a specific contact resistance of 8.9xc3x9710xe2x88x928 xcexa9cm2 is obtained, which is remarkably lower than the value obtained in the second prior art example. In this case, it is important that the Ni layer 104 and the Al layer 103 are thick. In addition, it is an indispensable condition for obtaining a low specific contact resistance that Ni and Au never diffuse into the n-type GaN contact layer 101. On the other hand, it was reported that when no annealing is conducted, the specific contact resistance is 3.3xc3x9710xe2x88x926 xcexa9cm2.
In the above mentioned prior art examples, a minimum specific contact resistance of 8.9xc3x9710xe2x88x928 xcexa9cm2 is obtained in the fourth prior art example. However, the annealing at as a high temperature as 900xc2x0 C. deteriorates other electrodes, semiconductor films and insulator films in the case that a semiconductor device is manufactured, and therefore, resultantly remarkably restricts a device manufacturing process. Accordingly, it is necessary to lower a necessary annealing temperature.
In addition, the fourth prior art reported that when no annealing is conducted, the specific contact resistance of 3.3xc3x9710xe2x88x926 xcexa9cm2 is obtained. The inventors actually manufactured the n-type contact electrodes in the same process as the fourth prior art example, and measured a contact characteristics of the n-type contact electrodes manufactured, However, no ohmic characteristics could be obtained when the annealing was conducted at a temperature of not greater than 400xc2x0 C. or when no annealing was conducted. This is because of damage on the surface of the n-type GaN contact layer 101 by the dry etching in the step shown in FIG. 4A, with the result that it is difficult to obtain the ohmic characteristics with good reproducibility, when the annealing is conducted at a temperature of not greater than 400xc2x0 C. or when no annealing is conducted. Furthermore, it is not preferred that damage remains on the contact layer when the annealing is not conducted.
Furthermore, in the electrode structures of the first and third prior art examples and in the electrode forming method of the second prior art example, no ohmic characteristics where obtained when the annealing was conducted at a temperature of not greater than 400xc2x0 C. or when no annealing was conducted.
Therefore, a technology for forming a low resistance n-contact electrode at a low annealing temperature is demanded.
In addition, when a large area for the n-contact electrode can be ensured, even if the specific contact resistance is not so low, if the ohmic characteristics of the n-contact can be obtained with good reproducibility with no annealing, the device manufacturing process can be simplified and also the degree of freedom in the manufacturing steps can be made large.
Accordingly, it is an object of the present invention to provide an n-type contact electrode having a low specific contact resistance and a method for forming the same, which have overcome the above mentioned defect of the conventional one.
Another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same with no annealing step.
Still another object of the present invention is to provide an n-type contact electrode having a low specific contact resistance, for a gallium nitride-based compound semiconductor and a method for forming the same by performing an annealing at a low temperature.
The above and other objects of the present invention are achieved in accordance with a first aspect of the present invention by a contact electrode having a further low specific contact resistance, including a high concentration oxygen-doped surface layer formed in a surface of an n-type contact layer of a gallium nitride-based compound semiconductor, and a metal electrode formed on the oxygen-doped surface layer.
The high concentration oxygen-doped surface layer can be formed for example by exposing the n-type contact layer of the gallium nitride-based compound semiconductor to an oxygen plasma. In this case, oxygen enters in the n-type gallium nitride-based compound semiconductor contact layer, a high concentration of oxygen donors are formed in a surface of the n-type gallium nitride-based compound semiconductor contact layer. Since good donors are formed in the surface of the n-type gallium nitride-based compound semiconductor contact layer, an n-type contact electrode having a low specific contact resistance is obtained with good reproducibility, with performing no annealing.
In an embodiment of the n-type contact electrode, the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer. Preferably, a Pt film is formed on the metal multilayer film.
According to a second aspect of the present invention, there is provided a method for forming a contact electrode, including the step of exposing an n-type contact layer of a gallium nitride-based compound semiconductor to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, and forming an electrode metal on the oxygen-doped surface layer.
In the method in accordance with the second aspect of the present invention, by exposing an n-type contact layer of a gallium nitride-based compound semiconductor to an oxygen plasma, oxygen donors are formed in a surface of the n-type gallium nitride-based compound semiconductor contact layer. By this processing, not only the oxygen-doped surface layer is formed in the surface of the n-type gallium nitride-based compound semiconductor contact layer, but also carbon is removed from the surface of the n-type gallium nitride-based compound semiconductor contact layer to which an electrode is to be formed. Therefore, an n-type contact electrode having a low specific contact resistance is obtained, and a good contact interface is obtained.
In the method in accordance with the second aspect of the present invention, after the oxygen-doped surface layer is formed by the oxygen plasma processing, an electrode metal is formed on the oxygen-doped surface layer. If an acid processing were conducted after the oxygen plasma processing as in the second prior art example explained hereinbefore, oxygen is removed, with the result that the oxygen donors are reduced, and therefore, an n-type contact electrode having a low specific contact resistance is no longer obtained. On the other hand, if an electrode metal is formed on the oxygen-doped surface layer with conducting no acid processing after the oxygen plasma processing but before formation of the electrode, even if no annealing is conducted, an n-type contact electrode having an ohmic characteristics is obtained with good reproducibility.
In one embodiment of the method in accordance with the second aspect of the present invention, after the electrode metal is formed on the oxygen-doped surface layer, an annealing can be conducted. By conducting the annealing, it is possible to obtain a specific contact resistance which is lower than that obtained when no annealing is conducted. A temperature for this annealing is preferably 500xc2x0 C. to 600xc2x0 C.
According to a third aspect of the present invention, there is provided a method for forming a contact electrode, including the step of dry-etching an n-type contact layer of a gallium nitride-based compound semiconductor, exposing the n-type contact layer to an oxygen plasma to form an oxygen-doped surface layer in a surface of the n-type contact layer, forming an electrode metal on the oxygen-doped surface layer, and thereafter, conducting an annealing. This annealing is conducted preferably at a temperature of 600xc2x0 C. to 800xc2x0 C.
In the method in accordance with the third aspect of the present invention, since the dry etching is conducted, crystal defect occurs in the surface region of the n-type contact layer. Therefore, differently from the method in accordance with the second aspect of the present invention, the ohmic characteristics cannot be obtained if the annealing is not conducted at a temperature of not less than 500xc2x0 C. However, if the annealing is conducted at a temperature of not less than 600xc2x0 C., it is possible to obtain a specific contact resistance which is further lower than that obtained in the method in accordance with the second aspect of the present invention. The reason for this is considered to be that because of the crystal defect generated by the dry etching, the alloying between the electrode and the contact layer is facilitated at the time of the annealing.
In an embodiment of the methods in accordance with the second and third aspects of the present invention, the metal electrode is formed of a metal multilayer film including a Ti layer in contact with the oxygen-doped surface layer and an Al layer formed on the Ti layer, or a metal multilayer film including a Ti/Al alloy layer in contact with the oxygen-doped surface layer. Preferably, a Pt film is formed on the metal multilayer film.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.